I thought I would report back!
When I posted this thread, I'd already bought a can of Dulux Trade Diamond Matt. I painted my study in this. The rest of the house has been done in the most expensive Valspar paint.
The Valspar is AWFUL. It marks like crazy - literally anything that touches it, anywhere, leaves a black mark. It even compares poorly to Dulux paintpod paint, which I had elsewhere in my house, and which is one of the thinnest paints that has been sold in the UK! The 'scrubbability' is a load of nonsense - try to clean it and you get a big smudge. It's cracking in places, and in other places it has been pulled off the wall with the yellow Frogtape, which is for delicate surfaces.
The Dulux, however, is fine. Looks fine, cleans fine, seems pretty robust.
The decorators are refusing to redo the job, so it looks like I'll be repainting the house in Dulux myself. They did, however, offer to come back and redo it for me in 12 months (!) Absolutely no chance of me ever employing them again, and in the process they've lost out on a load of additional work too, because my best mate owns a load of property in the city and was thinking of using them, but now will definitely not be doing so.